Em…I guess it is intended.
In version before, maybe before v1.005, things are like what you have speculated, any single attack in a aoe is roll seperately and will add a stack or consume all stack of fast adaption/head hunter . But after then only head hunter keeps the same, but fast adaption will not stack more than one stack in a aoe attack.
As it is changed intentionally, I guess iths working as intendes.